Travertine is a type of limestone deposited by hot mineral springs called karst. The water movement from the springs erodes the travertine, creating holes in the stones structure. The structure of travertine can be likened to that of a honeycomb, where small pockets or cavities exist within the stone; these are exposed as holes in the surface when it’s cut into tiles. These holes can either be left open or filled by the factory making the tiles with cement based filler. Occasionally resin filler is used especially in polished travertine.